In a semiconductor integrated circuit, not only an operating speed and a circuit area but also a power consumption is one of important factors, and the semiconductor integrated circuit has been demanded to achieve a low power consumption. In order to improve the power consumption of the semiconductor integrated circuit, there is a technique of performing a control in accordance with a process variation and a use state of the semiconductor integrated circuit. For example, there are techniques such as an ASV (Adaptive Supply Voltage) for controlling a power supply voltage supplied to a semiconductor integrated circuit, an ABB (Adaptive Body Bias) for controlling a back bias, and a DVFS (Dynamic Voltage Frequency Scaling) for dynamically controlling an operating frequency and a power supply voltage.
It is possible to appropriately conduct the control in accordance with the process variation and the use state of the semiconductor integrated circuit, by using a sense circuit and the like to read a state of the semiconductor integrated circuit. When the low power consumption technique as described above is employed, delay characteristic information of the semiconductor integrated circuit is important, and it is possible to determine an operation margin of the semiconductor integrated circuit and a tolerance value capable of controlling a speed of the semiconductor integrated circuit, from the delay characteristic information. As a method of obtaining the delay characteristic information of the semiconductor integrated circuit, a method of using a ring oscillator (refer to Patent Document 1, for example) and a method of using a buffer ring (refer to Patent Document 2, for example) have been known.
In the method of using the ring oscillator, the ring oscillator is configured by using a circuit to be measured, and a number of times of oscillation during a certain period is measured. From a frequency calculated based on the measured number of times of oscillation, a sum of a delay time when a low-level signal is input and a delay time when a high-level signal is input, is obtained as delay characteristic information of the circuit to be measured. The delay time when the low-level signal is input corresponds to a delay time when an input signal is changed from a high level to a low level. The delay time when the high-level signal is input corresponds to a delay time when an input signal is changed from a low level to a high level. However, in the method of using the ring oscillator, it is not possible to accurately divide the delay time when the low-level signal is input and the delay time when the high-level signal is input.
In the method of using the buffer ring, a plurality of buffers 200 being circuits to be measured are connected in series in a ring shape to configure the buffer ring, as illustrated in FIG. 8A. For example, a state where a node NDA in the buffer ring is fixed to a low level, and a state where a node NDB separated by a half round from the node NDA is fixed to a high level are created, and then the states are simultaneously released to start an oscillation. When a delay time when a low-level signal is input is shorter than a delay time when a high-level signal is input in the buffer 200 being the circuit to be measured, for example, if a signal is observed at one node in the buffer ring, a delay time DLL in the vicinity of the low level is shorter than a delay time DLH in the vicinity of the high level, as illustrated in FIG. 8B. As a result of this, a waveform is deformed and a high-level period becomes gradually short (refer to 210 in FIG. 8B), and after the oscillation is performed a certain number of times, the oscillation is stopped.
As described above, in the method of using the buffer ring, by measuring the number of times of oscillation up to when the oscillation is stopped due to the difference of the delay times, and by obtaining a signal level when the oscillation is stopped, it is possible to obtain a ratio of the delay time when the low-level signal is input and the delay time when the high-level signal is input, as delay characteristic information of the circuit to be measured. For example, when the oscillation is performed N times and the signal is eventually fixed to a high level, the high-level signal travels (N+½) rounds when the low-level signal travels N rounds in the buffer ring, resulting in that it can be understood that the speed of signal in the vicinity of the high level is faster than that in the vicinity of the low level, and a speed ratio of the low-level signal and the high-level signal is N:(N+½).
Patent Document 1: Japanese Laid-open Patent Publication No. 2010-87275
Patent Document 2: Japanese Laid-open Patent Publication No. 2011-166222
In the method of using the ring oscillator described above, it is possible to obtain, as the delay characteristic information of the circuit to be measured, the sum of the delay time when the low-level signal is input and the delay time when the high-level signal is input. In the method of using the buffer ring, it is possible to obtain, as the delay characteristic information of the circuit to be measured, the ratio of the delay time when the low-level signal is input and the delay time when the high-level signal is input. However, it is not possible to obtain a value of each of the delay time when the low-level signal is input and the delay time when the high-level signal is input, as delay characteristic information of the circuit to be measured.